This invention relates to colored polyester compositions and more particularly to colored polyester compositions derived from unpurified or crude terephthalic acid and a diol and containing a color-forming component.
It is known that linear thermoplastic polyester material such as polyethylene terephthalate can be prepared by direct esterification of terephthalic acid with a diol as well as by ester interchange of dimethylterephthalate with a diol. Preparation of polyester material by these two routes, however, is not without disadvantages. By the ester interchange route terephthalic acid must first be esterified to dialkylphthalates with alkyl alcohols and then subjected to the ester interchange reaction with an appropriate diol. The preparation of commercially acceptable polyester from terephthalic acid has generally required that terephthalic acid be of exceptionally high purity.
However, terephthalic acid produced by a common method, for example, by the oxidation of p-xylene with an oxygen containing gas (e.g., air) in the presence of a heavy metal oxidation catalyst, such as cobalt or manganese, as shown in U.S. Pat. No. 2,833,816 (1958), contains unacceptable levels of by-products and impurities, for example, 4-carboxybenzaldehyde, p-toluic acid and catalyst metal residue, which can interfere with attaining the desired properties in the polyester material. Because of the high levels of impurities resulting from such oxidation processes, extensive purification methods have been developed in an attempt to reduce their presence so as to attain the high quality terephthalic acid needed in the production of commercially suitable polyester material. (See, for example, U.S. Pat. Nos. 4,051,178; 3,657,330, 3,626,001; and 3,592,847.)
A commercially acceptable polyester material which can be produced from terephthalic acid without first subjecting it to extensive purification procedures would, therefore, be highly desirable.
A polyester composition has now been found which can be produced from unpurified terephthalic acid. The polyesters of this invention are colored linear thermoplastic polyesters which can be formulated into various articles, for example, films, sheets and hollow structures such as colored beverage bottles. Surprisingly, articles formulated of the polyester compositions derived from crude terephthalic acid exhibit good clarity and brightness, two properties which commercial standards generally require of polyester material used in articles such as colored beverage bottles. The polyester compositions of this invention are also advantageous in that reduced amounts of colorant are required for polyesters of selected colors as compared to polyesters of the same color which have been produced from purified terephthalic acid or dimethylterephthalate. In addition, the colored polyesters of the present invention can be produced more economically since the terephthalic acid need not first be purified or converted to a corresponding dialkyl ester as generally required heretofore, thus resulting in a reduction in production time, equipment and reagents.